How to I find the correct ci_val_id node for a specific value (e.g.: "E").

For example in this code:

Code:

var xml; // (is the XML from above)
var node = Ext.DomQuery.selectNode('ci_value:nodeValue("E")', xml);
// so far so good, node is pointing to the right node, now I want to know the ci_val_id for that record:
var ci_val_id_node = node.prev(); // This is where it goes wrong! prev() is not a valid function!